Generate a monomorphic version .createReadOnly for each class model

The VScode Deopt explorer showed that `.createReadOnly` inside the class
model `register` function was getting deoptimized because it closes over
too-wide a variety of `klass` variable references. We're closing over it
so it is correct, but to v8, the callsite is still the same I guess, so
it has to deoptimize to pull the right value for the closure!

Removing this deoptimization added about 10% to the large root benchmark
for me!

src/fast-instantiator.ts

@@ -79,6 +79,22 @@ class InstantiatorBuilder<T extends IClassModelType<Record<string, IAnyType>, an
         [$memos] = null;
         [$memoizedKeys] = null;
 
+        static createReadOnly = (snapshot, env) => {
+          const context = {
+            referenceCache: new Map(),
+            referencesToResolve: [],
+            env,
+          };
+      
+          const instance = new ${this.model.name}(snapshot, context, null);
+      
+          for (const resolver of context.referencesToResolve) {
+            resolver();
+          }
+      
+          return instance;
+        };
+
         constructor(
           snapshot,
           context,